This invention relates to snap acting heat motor operated gas valves, and particularly to an improved heat motor operator of the bimetal type for gas valves.
The gas valve of this invention is particularly useful in conjunction with an electric resistance igniter of the hot surface type in a gas burner system. In such systems, the electric heater of a heat motor operated gas valve is operated directly in series with an igniter such as a silicon carbide igniter which has a negative temperature coefficient of resistance. When electric power is applied to such a system, the resistance value of the igniter decreases as the igniter becomes heated and results in an increasing current flow through the series connected valve. Accordingly, the gas valve must be designed to open only when the current level reaches or exceeds a predetermined value at which the igniter is sufficiently heated to ignite the gas that is supplied upon opening of the valve. It is also essential that the gas valve open and close rapidly without any tendency to modulate open and closed even though the ambient temperature in which the valve operates may vary from normal room temperatures to temperatures as high as 300.degree. F. Because of variations in current characteristics and resistance values of igniters, it has been considered necessary to carefully adjust each heat motor operated gas valve to match the particular type of igniter with which it is used.
One type of heat motor operated gas valve used in gas burner systems is disclosed in the White U.S. Pat. No. 3,229,956 granted Jan. 18, 1966. The gas valves shown in this patent each utilizes a control bimetal actuator which is heated by an electric heater to move a valve member attached to a leaf spring carried by the bimetal actuator. The latter also carries an ambient temperature compensating bimetal which acts on the leaf spring to offset the effects of ambient temperature changes upon the control bimetal actuator. The valve member includes a flexible diaphragm element employed to promote a snap action motion of the valve member relative to its valve seat. The valves disclosed in this White patent do not have the critical current opening characteristics required for use in conjunction with a series connected silicon carbide igniter in a gas burner system. It will be apparent that the current value required to open such a normally closed valve will be affected by the pressure of the gas supplied to the valve. Although, such a valve has a snap action opening motion when energized at suitable current levels, it has a tendency to modulate open and closed when energized at lower current levels.
It is known, of course, that snap action operation of a gas valve may be obtained by employing a magnet arrangement. The Houser U.S. Pat. No. 3,227,370 granted Jan. 4, 1966 and the Kelly et al. U.S. Pat. No. 3,876,137 granted Apr. 8, 1975, for example, each disclose a gas valve of this type. However, a magnet arrangement cannot be satisfactorily employed with the gas valves disclosed in the aforesaid White patent. In these valves, the plane of the valve member support departs substantially from the plane of the valve seat with increasing ambient temperatures and thus would act to separate the magnet and armature of a magnet arrangement at higher ambient temperatures.
Another type of heat motor operated gas valve used in gas burner systems is disclosed in the Perl U.S. Pat. No. 4,102,496 granted July 25, 1978. The gas valves shown in that patent each utilizes a control bimetal actuator which is heated by an electric heater to operate an over-center spring mechanism for effecting snap opening and closing of a valve member. The use of a spring in valves of this type is disadvantageous since it may become fatigued after some use and cause the operating characteristics of the valve to vary. Furthermore, such gas valves are relatively expensive to manufacture and their costs are prohibitive for use of the valves in many gas burner systems.